The present invention relates to a gas detecting apparatus using laser absorption spectroscopy, and, more particularly, to a gas detecting apparatus having a gas cell which brings about the interaction of a gas to be supplied therein and a laser beam in order to detect the presence or absence of gas as a detection target and measure the concentration of the gas.
It is known that laser absorption spectroscopy is one kind of spectroscopy technology which is used to analyze a target substance using such a property that a substance demonstrates significant absorption with respect to a specific optical wavelength.
Applying this spectroscopy technology in the field of gas analysis can ensure detection of a target gas and measurement of the concentration of that gas.
FIG. 9 exemplifies an optical system in a conventional gas detecting apparatus.
This optical system comprises a laser source 1, a gas cell 2 where a laser beam 4 from the laser source 1 enters and into which a gas as a detection target is fed, as will be discussed later, and a photodetector 3 which receives the laser beam from this gas cell 2.
The gas cell 2 generally has a container 5 for retaining a gas, a gas inlet hole 8, a gas discharge hole 9, and first and second windows 6 and 7 through which the laser beam 4 passes.
The laser beam 4 emitted from the laser source 1 enters the gas cell 2 through the first window 6, and is then partially absorbed by the gas retained in the container 5, so that the remaining portion of the laser beam 4 emerges from the second window 7 to be received by the photodetector 3.
Spectroscoping the laser beam 4 received by the photodetector 3 by unillustrated spectroscope means, i.e., changing the wavelength of the laser beam 4 in the vicinity of the absorption wavelength of the target gas yields a gas absorption characteristic curve as shown in FIG. 10.
The concentration of the target gas can be detected by measuring the degree of absorption of the laser beam by the gas or the amount of change in the quantity of the laser beam received by the photodetector 3 using a laser beam whose wavelength is as close as possible to the wavelength that shows large absorption on the gas absorption characteristic curve.
When the concentration of a gas is actually measured using the optical system of such a gas detecting apparatus, errors occur due to various factors. Of those errors, the drift-originated error that occurs by the coherence of the laser beam has a large absolute value and is difficult to cope with.
Specifically, when the laser beam 4 passes the windows 6 and 7 of the gas cell 2 or passes through the optical system in the photodetector 3, multiple reflection occurs and the multiple-reflected light component interferes with the original laser beam, so that the result appears as a variation in light intensity.
When this variation in light intensity is superimposed on the absorption curve, it is observed as if the concentration of the target gas were changed.
To avoid this optical multiple reflection as much as possible, a countermeasure as shown in FIG. 11 has been taken conventionally (see FIG. 2 and the description lines 15 to 18 of the left column on page 4028, in APPLIED OPTICS/vol. 35, No. 21/20 July 1996, pp. 4026-4032).
In this example, a wedge-like glass plate with a reflection prevention film 12 formed on either side of the glass plate is used for the windows 6 and 7 of the gas cell 2.
This scheme can reduce the influence of the coherence that is caused by the aforementioned multiple reflection.
Even if multiple reflection is suppressed in the gas cell 2 as shown in FIG. 11, a similar problem arises in the photodetector 3 due to coherence of the laser beam.
When a photodiode (hereinafter referred to as xe2x80x9cPDxe2x80x9d) is used as the photodetector 3, for example, multiple reflection occurs between the window of the PD container and the surface of the PD. The multiple-reflected light component leads to a measuring error.
Because a coherence-originated error may occur at various places in the optical system, it is hard to make such an improvement as not to cause coherence in every optical part.
Accordingly, it is an object of the present invention to provide a gas detecting apparatus which utilizes laser absorption spectroscopy and can measure the concentration of a target gas to be detected at a high precision by suppressing a measuring error caused by multiple reflection of a laser beam used in the measurement.
To achieve the above object, a gas detecting apparatus according to one aspect of this invention comprises:
a gas cell for retaining a target gas to be detected, the gas cell having a gas inlet hole for letting the target gas come inside and a gas discharge hole for discharging the target gas;
a laser source for emitting coherent light into the gas cell in order to detect a concentration of the target gas;
light diffusion means, disposed in an optical path of the coherent light output from the laser source, for diffusing the coherent light, thereby eliminating coherence of the coherent light; and
a photodetector for receiving light diffused by the light diffusion means.
According to this aspect of the invention, providing the light diffusion means inside or outside the gas cell can reduce optical noise which is originated from coherence.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.